1. Field of the Invention
This invention relates to a printing apparatus including an exposing head having a plurality of light control elements arranged in a primary scanning direction, the exposing head and a photosensitive material being movable relative to each other in a secondary scanning direction perpendicular to the primary scanning direction, the light control elements being controllable, in time of the relative movement, based on set exposure data to create a test chart; a scanner for scanning the test chart to acquire scanning data; and a correction data generating means for setting correction data to the light control elements based on the scanning data.
2. Description of the Related Art
In an existing printing apparatus having the above construction, a test print sheet (corresponding to the test chart of this invention) is created from printing paper serving as the photosensitive material exposed by a fluorescent print head (corresponding to the exposing head of this invention), a flatbed scanner reads line images from this test print sheet, and a correction table is set based on the density of exposed dot lines read. The correction table obtained in this way is used as data for correcting variations in the luminance of light emitting elements arranged on the exposing head. See Japanese Patent Publication “Kokai” No. 2001-142162 (paragraphs [0011] to [0031] and [0037], and FIGS. 1-15), for example.
In the prior art described in this publication, even numbers and odd numbers are assigned to fluorescent light emitting elements corresponding to the three primary colors of R (red), G (green) and B (blue) constituting the fluorescent print head. These fluorescent light emitting elements are controlled to expose printing paper, thereby to create a test print sheet. The test print sheet has a plurality of test exposed dot lines formed thereon and corresponding to the fluorescent light emitting elements having the even numbers and odd numbers assigned thereto. Then, density is determined of middle positions in a primary scanning direction of the exposed dot lines (i.e. direction of width of the exposed dot lines) acquired by a flatbed scanner, which middle positions are present at predetermined intervals in a secondary scanning direction. Further, an average value of density data in a check window is regarded as a representative density, and a correction coefficient for each fluorescent light emitting element is generated from the representative density and stored in a correction table.
In the above conventional technique, information is acquired from the test print sheet with the flatbed scanner, and densities are acquired from pixels present in the secondary scanning direction and on virtual center lines set to the middle positions in the primary scanning direction of the exposed dot lines (i.e. direction of width of the exposed dot lines). A correction table is set from average values of the densities of the plurality of locations acquired in this way.
However, when scanning the test chart with the scanner to set correction data, improper correction data may be set as a result of acquiring density data from exposed dot lines deviating from target exposed dot lines. This may be caused by a phenomenon of the exposing head and photosensitive material not moving relative to each other with high accuracy when creating the test chart by emitting light from the plurality of light control elements of the exposing head to the photosensitive material. When this phenomenon occurs, the exposed dot lines formed on the test chart are inclined relative to the secondary scanning direction (which is perpendicular to the primary scanning direction). Thus, when sampling points are set in positions spaced in the secondary scanning direction from ends of the exposed dot lines, the sampling points are displaced in the primary scanning direction with reference to proper exposed dot lines, leading to the phenomenon of measuring the density of adjoining exposed dot lines.
This phenomenon may be caused by a situation where the exposing head, which is the movable type, does not move accurately in the direction (secondary scanning direction) perpendicular to the primary scanning direction (i.e. the direction of arrangement of the light control elements), but moves in a direction at an angle, small as it is, to the secondary scanning direction. The test chart created by exposing action of the exposing head moving in the manner described above has the exposed dot lines inclined relative the secondary scanning direction as noted above. There is room for improvement in that proper correction data cannot be generated. This inconvenience arises also with a printing apparatus in which the exposing head is fixed and the photosensitive material is moved for exposure, and when the photosensitive material is not transported properly toward the secondary scanning direction of the exposing head.